1. Field of the Invention
The present invention is in the field of vacuum interrupters generally and is particularly directed to the composition of a vacuum interrupter electrical contact.
2. Description of the Prior Art
In a vacuum interrupter the current normally flows between and through a pair of electrical contacts located within an evacuated envelope. These contacts are relatively movable between a closed position in engagement and an open position spaced apart to interrupt the current flow through the vacuum interrupter. As the contacts are separated an arc is formed therebetween and current will continue to flow until this arc is extinguished, which on an alternating current circuit will normally occur near the first current zero. The contact surface must support the arc from its initiation at the time of contact separation until its extinction at approximately current zero. While the arc is being sustained the contacts are subjected to very intense localized heating. The dissipated arc energy causes melting, erosion and general deterioration of the contact surfaces. To minimize deterioration of the contact surfaces, it is customary to move the arc around the contact surface. Such arc movement tends to minimize the amount of metallic vapors or particles generated by the arc from the contact surfaces during circuit interruption. This arc movement is usually accomplished by self-induced magnetic fields as more fully explained in prior art U.S. Pat. No. 3,089,936 issued May 14, 1963 to S. R. Smith, Jr. and U.S. Pat. No. 3,417,216 issued Dec. 17, 1968 to S. R. Smith, Jr.
One prior art method for moving the arc around the contact surface is to form slots in at least a portion of the contacts.
As pointed out in U.S. Pat. No. 3,087,936 Supra these slots may extend to the mating surfaces of the opposed contacts or as pointed out in U.S. Pat. No. 4,149,050 issued Apr. 10, 979 to Gorman et al., as in the present invention, the contact may have a cup-shaped holder portion in which the slots are formed and a contacting portion affixed to the holder portion and which mates with the contacting portion of the opposed contact.
In either case, as pointed out in U.S. Pat. No. 4,390,762 issued June 28, 1983 to Watzke, the slots result in decreasing the overall strength of the contact. The contacts are partially compressed by the switching load in the course of the contacts life. As a result of the compression, the stroke of the vacuum interrupted must be adjusted after as few as 300 or even less switchings.
Attempts have been made to reinforce the slotted portion of the contacts.
Watzke, Supra, teaches reinforcing the slotted portion of the contact with a steel ring member disposed concentrically within the slotted portion.
British Pat. No. 1,142,209 teaches disposing a plurality of reinforcing strips of a resilient metal, as for example molybdenum or a refractory metal, in the slotted portion of the contact. The strips run parallel to the vertical axis of the contact. British Pat. No. 1,100,259 also is directed to reinforcing strips.
British Pat. No. 1,129,152 teaches making the contacts out of a copper-iron alloy, 1% to 5% iron, by weight, to prevent or at least to retard compression.
The proposed prior art solutions all suffer from the same shortcoming. The conductivity of the contact is reduced relative to copper by the addition of the reinforcing metal and as a result the temperature of the current carrying lead is increased during the current conduction period.